1. Field of the Invention
This invention relates generally to a method and apparatus for detecting the presence of a transmitter, particularly a covert eavesdropping audio transmitter, by detecting the radio frequency signal it broadcasts.
2. Description of the Prior Art
A typical audio surveillance apparatus or "bug" constitutes a transmitter operating on an R.F. carrier. The sounds picked up by the bug's microphone modulate the carrier and are transmitted to a remote location at which the bug is being monitored. When a signal received during the operation of a radio receiver, used, for example, to detect bugs, is suspected of coming from such a bug, the signal must be identified by examining the signal either visually or aurally for indications that it is being modulated by sounds in the room that is being searched. Identification of such a signal may be difficult because the signals transmitted by the bugs may often be weak and have a poor signal to noise ratio. One way to overcome that problem is to move the receiver until it receives the signal more clearly. In some situations, however, it may not be practical to move the receiver about, and in other situations such movement may not produce the desired effect.
Several methods and devices for detecting the R.F. signal emitted by the transmitter of a covert eavesdropping device are known in the prior art. For example, in U.S. Pat. No. 3,473,127 to Williams and U.S. Pat. No. 4,127,817 to Bell, the intercepted signal is demodulated and channeled by means of a loud speaker into the area being checked. If the signal in question emanates from an eavesdropping device, the device will pick up its own signal, thereby causing a feedback squeal. This system is not advantageous, however, for this feedback squeal unavoidably alerts the eavesdropper that detection efforts are being undertaken. The eavesdropper may thus turn off the eavesdropping device, thereby making the exact location of the bug and the listening location impossible to determine electronically.
Another method for detecting the presence of a bug involves using a search signal chosen so as to minimize the probability of the simultaneous occurrence of the search signal frequency in talk and music. The search signal is transmitted within the confined space suspected of containing a bug, and if a bug is present, the search signal will energize the microphone of the bug, thereby causing it to transmit a radio frequency signal which is modulated by the search signal. The radio frequency is then demodulated in the receiver, and the resulting demodulated signal is applied to a narrow band pass filter which is tuned to the search signal frequency. The output of the filter is sensed to determine if the bug is present. Such a method and device for implementing it are disclosed in U.S. Pat. No. 3,939,420 to Risberg et al. This method is also disadvantageous because the search signal may alert the eavesdropper that a hunt is underway for the bug. Also, as in the feedback method, the generation of such an audible signal in the area to be searched makes the continued operation of these devices during the course of confidential communications impractical. Similar problems exist with detection devices disclosed in U.S. Pat. No. 4,501,020 to Wakeman, and U.S. Pat. Nos. 4,399,556, 4,368,539 and 4,264,978 all to the present inventor.